1. Field of the Invention
This invention relates generally to fused salt bath tanks, and more particularly to such tanks and a method of treating materials in a bath for improved settling and handling of the sludge generated.
2. Description of the Prior Art
One of the problems encountered in the process of cleaning and descaling of material in fused salt baths is the control and handling of the sludge generated by the reaction of the metal scale or other material such as paint, grease, etc. on the surface of the metal with the fused salt. This sludge normally is in the form of a fairly fine, inert granular material. When the salt bath is in a relatively quiescent, non-agitated condition, this sludge normally will settle to the bottom. However, the bath cannot normally be maintained quiescent since it is necessary to recirculate the bath past heating elements which heat the salt to maintain the temperature. During conventional recirculation, the sludge tends to stay in suspension dispersed throughout the bath due to the agitation of the recirculation. As the sludge builds up, it tends to interfere with the reaction of the material of the salt bath with the metal scale or other material on the surface of the work piece, thus reducing the efficiency of the bath action. Historically, one way to deal with this problem was to periodically stop the agitation of the recirculation and allow the sludge to settle to the bottom into a sludge pan. The pan is then removed, the sludge dumped, the pan returned, and then agitation of the bath resumed.
This method had serveral drawbacks including the accumulation of sludge during the process with attendant decreasing bath efficiency, and the necessity to shut the process down for desludging which interrupts production. Also, when the agitation is stopped, the sludge will also settle out in the region of the heating elements. These elements are not easily removable thus making more difficult the recovery of the settled sludge from the region of the heating elements. One prior art solution to this problem was to provide a separate sludge collection zone in the bath tank. This zone was maintained relatively quiescent, with moderate laminar quiescent flow of the fused salt in the sludge collection zone which allowed continuous settling of the sludge. However, this practice has a drawback in that an entire section of the tank must be dedicated solely to sludge collection, thus necessitating extra salt and with attendant additional energy costs for heating, as well as the other costs associated with larger tanks and baths.